The present invention relates to a rubber composition blended with silica, which is improved in a dispersibility of silica into rubber, a heat generating property and an abrasion resistance, and a pneumatic tire using said rubber composition.
Carbon black has so far been used as a reinforcing filler for rubber. This is because carbon black has both a high reinforcing property and a high abrasion resistance as compared with other fillers. In recent years, under the circumstances of social requirements for saving energy and natural resources, particularly in order to save fuel consumption of automobiles, a rubber composition has come to be required to have a low heat generating property as well.
In the case of intending to provide a low heat generating property to a rubber composition by virtue of carbon black, it is considered to blend carbon black in a small quantity or to use carbon black having a large particle size. In either method, however, it is well known that a low heat generating property has an antinomic relation with a reinforcing property and an abrasion resistance.
On the other hand, silica is known as a filler for a rubber composition providing a low heat generating property, and many patents such as Japanese Patent Application Laid-Open No. Hei 3-252431 have so far been filed.
Silica, however, tends to cause the particles thereof to coagulate due to a hydrogen bond between silanol groups which is a functional group on the surface, and therefore the mixing time has to be extended in order to improve the dispersion of silica particles into rubber. Further, silica has had defects that since the dispersion of silica particles into rubber is insufficient, the Mooney viscosity of the rubber composition rises and processability, for example, extrusion is inferior.
Further, silica has had a defect as well that since a silica particle has an acidic surface and adsorbs a base material used as a vulcanization accelerator, the rubber composition is not sufficiently vulcanized and therefore the modulus of elasticity is not increased.
A silane coupling agent has been developed in order to improve these defects, but the dispersion of silica has not yet reached a sufficiently high level. In particular, it has been difficult to obtain the industrially sufficient dispersion of silica.
Further, it is disclosed in Japanese Patent Application Laid-Open No. Hei 5-51484 to blend silylation agents in order to improve the dispersibility of silica. The silylation, however, has the defects that silica has to be reacted with the silylation agents in a short time of mixing and therefore the reaction efficiency is not sufficiently high and that these silylation agents have low boiling points and are volatilized during mixing and therefore the reaction does not go on sufficiently.
Further, it is disclosed in Japanese Patent Publication No. Sho 63-2886 and Japanese Patent Application Laid-Open No. Hei 6-157825 to use hydrophobic precipitated silicic acid. In those cases, however, there have been the defect that since precipitated silicic acid subjected to complete hydrophobic treatment is used, silanol groups on the surface with which a silane coupling agent is to react have been lost and therefore the rubber is not sufficiently reinforced.
On the other hand, disclosed in Japanese Patent Application Laid-Open No. Hei 3-197536 is a rubber composition having an improved heat generating property, in which blended are 20 to 150 parts by weight of a reinforcing filler such as carbon black, and 0.05 to 20 parts by weight of an alkylamine compound per 100 parts by weight of at least one rubber selected from the group consisting of natural rubber and diene base synthetic rubber.
In Japanese Patent Application Laid-Open No. Hei 3-197536 described above, the alkylamine compound is blended in order to improve the heat generating property. However, it neither described nor suggested that the dipersibility of silica as well as a heat generating property and an abrasion resistance is improved in a rubber composition containing silica.
The present invention is intended to solve the conventional problems described above, and an object thereof is to provide a rubber composition which is improved in a dispersibility of silica into rubber, a heat generating property and an abrasion resistance, and to provide a pneumatic tire using the same.
Intensive researches made by the present inventors in order to solve the problems described above have resulted in successfully obtaining a rubber composition which is improved in dispersion of silica into rubber to a large extent and is provided with a low heat generating property and a high abrasion resistance by mixing a specific dispersion improving agent with a prescribed amount of silica, and a pneumatic tire using said rubber composition, and thus coming to complete the present invention.
That is, the rubber composition and the pneumatic tire of the present invention resides in the following items (1) to (7):
(1) A rubber composition comprising 15 to 85 parts by weight of silica per 100 parts by weight of a rubber component comprising natural rubber and/or diene base synthetic rubber and a tertiary amine compound represented by the following Formula (I) blended in a proportion of 1 to 15% by weight based on the amount of silica described above: 
wherein R1 represents an alkyl group, alkenyl group or alkoxyl group having 1 to 36 carbon atoms and hydroxy-substituted group thereof, a benzyl group, a benzyl group substituted with an alkyl group or alkenyl group having 4 to 36 carbon atoms, or a group represented by one of the following Formulas (II); and R2 and R3 independently represent an alkyl group or alkenyl group having 1 to 36 carbon atoms, a benzyl group, a cyclohexyl group and a hydroxy-substituted group thereof: 
wherein R4 represents an alkyl group or alkenyl group having 1 to 36 carbon atoms; R5 represents an ethylene group or a propylene group; X represents hydrogen, an alkyl group, alkenyl group, alkanoyl group or alkenoyl group having 1 to 18 carbon atoms; A represents an alkylene group or hydroxyalkylene group having 2 to 6 carbon atoms; m is an integer of 1 to 10, and when m is 2 or more, respective A may be the same or different; and n is an integer of 1 to 10.
(2) The rubber composition as described in the above item (1), wherein the tertiary amine compound represented by the Formula (I) described above has a molecular weight of 180 or more.
(3) The rubber composition as described in the above item (1), wherein the tertiary amine compound represented by Formula (I) described above is dimethylalkylamine in which R1 and R2 are both a methyl group and R3 is an alkyl group having 12 to 36 carbon atoms.
(4) The rubber composition as described in the above item (1), wherein the tertiary amine compound represented by the Formula (I) described above is dimethylstearylamine.
(5) The rubber composition as described in the above item (1), further comprising a silane coupling agent blended in a proportion of 1 to 15% by weight based on the amount of silica.
(6) The rubber composition as described in the above item (1), further comprising carbon black of 20 to 80 parts by weight per 100 parts by weight of the rubber component.
(7) A pneumatic tire comprising a tread part, a side wall part and a bead part, wherein the above tread part comprises the rubber composition as described in any of the items (1) to (6).
The embodiment of the present invention shall be explained below in detail.
Natural rubber (NR) or synthetic rubber can be used alone or by blending them as a rubber component in the present invention. The synthetic rubber includes, for example, synthetic polyisoprene rubber (IR), polybutadiene rubber (BR) and styrene-butadiene rubber (SBR).
For example, precipitated silica is preferred as silica used in the present invention, but no specific restrictions shall be put thereon. The blend amount of silica is 15 to 85 parts by weight per 100 parts by weight of the rubber component described above. If the blending amount of silica is less than 15 parts by weight, the sufficient reinforcing property is not exerted, and if the blending amount exceeds 85 parts by weight, the processability such as warming up and extrusion is deteriorated. The blending amount of silica is preferably 20 to 65 parts by weight in terms of the reinforcing property, the low heat generating property and the processability.
The amine compound used as a dispersion improving agent of the present invention is the tertiary amine compound represented by the following Formula (I): 
wherein R1 represents an alkyl group, alkenyl group or alkoxyl group having 1 to 36 carbon atoms and hydroxy-substituted group thereof, a benzyl group, a benzyl group substituted with an alkyl group or alkenyl group having 4 to 36 carbon atoms, or a group represented by one of the following Formulas (II); and R2 and R3 independently represent an alkyl group or alkenyl group having 1 to 36 carbon atoms, a benzyl group, a cyclohexyl group and a hydroxy-substituted group of thereof: 
wherein R4 represents an alkyl group or alkenyl group having 1 to 36 carbon atoms; R5 represents an ethylene group or a propylene group; X represents hydrogen, an alkyl group, alkenyl group, alkanoyl group or alkenoyl group having 1 to 18 carbon atoms; A represents an alkylene group or hydroxyalkylene group having 2 to 6 carbon atoms; m is an integer of 1 to 10, and when m is 2 or more, respective A may be the same or different; and n is an integer of 1 to 10.
To be specific, R1 includes methyl, ethyl, propyl, lauryl, stearyl, laurolyaminoethylene, stearoyloxyethylene, acryloyloxypropylene, methacryloyloxypropylene, 2-hydroxymethyl, 2-hydroxydodecyl, benzyl and cyclohexyl groups.
To be specific, R2 and R3 include methyl, ethyl, lauryl, stearyl, vinyl, allyl, 3-allyoxy-2-hydroxypropyl, benzyl and hydroxyethyl groups.
To be specific, R4 includes methyl, ethyl, lauryl, stearyl, vinyl and allyl groups.
To be specific, X includes a hydrogen atom, ethyl, lauryl, stearyl, vinyl, allyl, lauroyl, stearoyl, acryloyl and methacryloyl groups.
The specific examples of the tertiary amine compound used in the present invention include trioctylamine, dimethyldecylamine, dimethylmyristylamine, dimethyloctadecenylamine, dimethylhexadecenylamine, dimethylmethacryloxypropylamine, methyldivinylamine, N,N,N-trilaurylamine, N,N,N-tristearylamine, N,N-dimethyl-N-laurylamine, N,N-dimethyl-N-stearylamine, N-methyl-N,N-dilaurylamine, N-methyl-N,N-distearylamine, N,N-dibenzyl-N-stearylamine, N-benzyl-N,N-dilaurylamine, N,N-diallyl-N-stearylamine, N,N-diallyl-N-laurylamine, N,N-dimethyl-N-lauryloyloxyethylamine, N,N-dimethyl-N-stearoyloxyethylamine, N,N-dimethyl-Nxe2x80x2-lauroylpropylamine, N,N-dihydroxyethyl-N-stearylamine, N,N-dihydroxyethyl-N-laurylamine, N,N-dihydroxyethyl-N-2-hydroxydodecylamine, N,N-polyoxyethylene-N-stearylamine, N,N-di(-2-hydroxy-3-allyloxypropyl)-N-hexadecylamine, N,N-di(-2-hydroxy-3-allyloxypropyl)-N-octadecylamine, N,N-di(-2-hydroxy-3-acrylyloxycarbonyl)propyl-N-hexadecylamine, N,N-di(-2-hydroxy-3-acrylyloxycarbonyl)propyl-N-octadecylamine, N,N-di(-5-hydroxy-3,7-dioxy-9-decyl-1-yl)-N-octadecylamine, and esterified compounds with acrylic acid, methacrylic acid and fatty acids.
Preferably, the tertiary amine compound is a dimethylalkylamine in which R1 and R2 are both a methyl group and R3 is an alkyl group having 12 to 36 carbon atoms, more preferably dimethylstearylamine from the viewpoint of a flash point, a low heat generating property and an improvement in dispersion.
The blending amount of the tertiary amine compound described above has to be 1 to 15% by weight, preferably 3 to 10% by weight based on the amount of silica. If the blending amount of the tertiary amine compound described above is less than 1% by weight, the intended dispersibility, low heat generating property and abrasion resistance can not sufficiently be improved. On the other hand, the blending amount exceeding 15% by weight saturates the dispersion improving effect on silica and causes the tertiary amine compound to act as a plasticizer, so that the abrasion resistance is lowered (this matter shall further be explained in detail in the examples).
The tertiary amine compound used in the present invention has preferably a molecular weight of 180 or more. If the tertiary amine compound has a molecular weight lower than 180, the flash point is low, and there is a risk of ignition during the processing steps. Accordingly, it is not preferred.
The silane coupling agent used in the present invention is not specifically restricted as long as it is usually used in the rubber industry. To be specific, it includes bis(3-triethoxysilylpropyl)polysulfide, xcex3-mercaptopropyltriethoxysilane, xcex3-aminopropyltriethoxysilane, N-phenyl-xcex3-aminopropyltrimethoxysilane and N-xcex2-(aminoethyl)-xcex3-aminopropyltrimethoxysilane. The blending amount of the silane coupling agent is preferably 1 to 15% by weight, more preferably 5 to 12% by weight based on the weight of silica. If the blending amount of the silane coupling agent is less than 1% by weight, the coupling effect is small, and the blending amount exceeding 15% by weight brings about gelation of the polymer. Accordingly, both are not preferred.
SAF,ISAF and HAF can preferably be used as carbon black used in the present invention, but it shall not specifically be restricted. The blending amount of carbon black is preferably 20 to 80 parts by weight per 100 parts by weight of the rubber component described above from the viewpoint of the reinforcing property and the low heat generating property. The blending amount of carbon black exceeding 80 parts by weight deteriorates the heat generating property to a large extent. The blending amount of 25 to 60 parts by weight provides the marked effects.
In the present invention, compounding agents usually used in the rubber industry such as softening agents, antioxidants, vulcanizing agents, vulcanization accelerators and vulcanization accelerating auxiliaries as well as the rubber component, silica, the tertiary amine compound, the silane coupling agent and the carbon black each described above can suitably be blended if necessary.
The rubber composition and the pneumatic tire of the present invention can be produced by mixing the rubber component, silica, the tertiary amine compound, the silane coupling agent and the carbon black each described above and the above rubber compounding agents blended with necessity by means of a banbury mixer and the like.
It is presumed that the rubber composition and the pneumatic tire of the present invention are improved in a dispersibility of silica into rubber to a large extent as well as a heat generating property and an abrasion resistance by the following action mechanism.
Usually, silica causes the particles thereof to coagulate due to a hydrogen bond of a silanol group which is a functional group on the surface and comes to be inferior in dispersion in rubber.
Further, the silane coupling agent usually used in the rubber industry reduces the number of silanol groups present on the surface of a silica particle by a dehydration-condensation reaction of the silanol group with a silanol group produced by hydrolysis of an alkoxyl group of the silane coupling agent to improve the dispersion of silica in rubber. This reaction is, however, difficult to take place at low temperatures and considered to proceed at temperatures of 140xc2x0 C. or higher. On the other hand, a three dimensional cross-linking reaction of rubber is brought about by the silane coupling agent at high temperatures of 170xc2x0 C. or higher, and the viscosity goes up drastically. In practice, temperatures of mixing rubber go up quickly at a rubber mixing step, and therefore a sufficiently long time for reaction between the silane coupling agent and silica can not be spent.
On the other hand, it is considered that the nitrogen atom in the tertiary amine compound described above has a high capacity of forming a hydrogen bond with a silanol group present on the surface of silica to exert the masking effect on the silanol group present on the surface of silica, thereby preventing the silica particles from coagulating each other. This reaction is not a primary bond but chemical adsorption, and therefore this effect can be displayed in a low temperature region close to room temperature.
Accordingly, an effect for preventing the silica particles from coagulating is exerted from the beginning of mixing rubber in a low temperature region. As a result, it is presumed that the rubber composition and the pneumatic tire of the present invention are improved in a dispersibility of silica into rubber to a large extent as well as a heat generating property and an abrasion resistance.